This project is directed to the identification and evaluation of the determinants of movement of pharmacologically active weak electrolytes across the walls of the alimentary tract and other epithelia. Unidirectional fluxes of weak electrolytes are evaluated in isolated systems in vitro with the objective of identifying the driving forces and principal resistances which determine net movements. In the third year of the project particular attention will be dircted to the transport of narcotic amines and narcotic antagonists in the small intestine, and to the transport of simple weak electrolytes in a newly developed preparation of mammalial gastric mucosa.